A metallic shell of a spark plug has a center electrode and a ground electrode which are positioned so as to face each other having a space, called a spark gap, therebetween. Recently, in order to further improve fuel consumption, the ground electrode is joined to the metallic shell in an optimal igniting position when the spark plug is mounted on an internal combustion engine with a specified torque. Notably, the “optimal igniting position” means a location where a growth of flame kernel generated by spark discharge in the spark gap is not interfered by airflow of an air-fuel mixture in a combustion chamber.
Generally, in order to check a joint position of a ground electrode, a female thread jig assuming the same shape as a female thread of a cylinder head of an internal combustion engine for mounting a spark plug is employed. In the situation where the metallic shell is engaged with a female thread, an allowable angle range for joining the ground electrode is indicated in such a female thread jig. The allowable angle range means an allowable deviation (angle) when the ground electrode is deviated from the optimal igniting position. A person who checks the deviation of the ground electrode engages the metallic shell with the female thread jig with the same torque as that used for mounting a spark plug on a cylinder head. Then, the person conducts a visual inspection to see whether or not the location of the ground electrode is in the allowable angle range and inspect quality of a product (metallic shell).